1. Field of the Invention
The present invention relates to a D/A converter circuit, an organic EL drive circuit and an organic EL display device. Particularly, in a current drive circuit for generating a drive current in a column direction or a current, from which the drive current is generated, by D/A converting display data corresponding to column pins of an organic EL panel, the present invention relates to an improvement of an organic EL drive circuit and an organic EL display device, which are capable of restricting luminous irregularity of a display screen due to irregularity of conversion characteristics of a D/A converter circuit.
2. Description of the Related Art
An organic EL display panel of an organic EL display device for use in a portable telephone set, a PHS, a DVD player, PDA (portable digital assistances), etc., including 396 (132×3) terminal pins (column pins) for column lines and 162 terminal pins for row lines has been proposed. These numbers of the terminal pins for column lines and row lines are still increasing.
JP2003-234655A assigned to the assignee of this application and U.S. Pat. No. 6,756,738 also assigned to the assignee of this application disclose drive circuits for corresponding column pins of such organic EL display panel, in each of which a D/A converter circuit is provided. In U.S. Pat. No. 6,756,7378, the D/A converter circuits are responsive to display data and a reference current to generate drive current in a column direction or a current on which the drive current is derived, by D/A converting the display data correspondingly to the column pins of the organic EL display panel according to the reference current. Particularly, when the organic EL display panel is of the passive matrix type, peak currents are generated to drive organic EL elements having capacitive load characteristics by initially charging the elements.
FIG. 6 is an example of a D/A converter circuit provided corresponding to column pins of a drive circuit of an organic EL display panel and constructed with MOS FETs mainly and corresponds to FIG. 1 of U.S. Pat. No. 6,756,738.
The D/A converter circuit 11 of the current drive circuit is the so-called a current switching D/A converter circuit and is constructed with a current mirror circuit. The D/A converter circuit 11 includes an input side transistor TNa to which a reference current Ip is inputted from a constant current source 12 through an input terminal 11a and output side transistors TNb˜TNn−1 to which display data D0˜Dn−1 are supplied from a register 16. The input side transistor TNa and the output side transistors TNb˜TNn−1 constitute a current mirror circuit. The D/A converter 11 multiplies the reference current Ip with the display data. In order to generate peak current, the D/A converter 11 further includes an N channel MOS transistor TNp connected in parallel to the transistor TNa.
A gate and a drain of the MOS transistor TNp are connected to the input terminal 11a. Sources of the transistor TNa and the MOS transistor TNp are grounded through resistors Ra and Rpa and switch circuits SWa and SWpa, respectively. The switch circuits SWa and SWpa are ON/OFF controlled by pulse signals P and CONT from a control circuit 15. In response to the reference current Ip, the D/A converter circuit 11 generate the peak current when the switch circuits SWa and SWpa are turned ON and OFF, respectively. When both of the switch circuits SWa and SWpa are turned ON, the D/A converter circuit 11 outputs the constant current corresponding to the reference current.
Incidentally, resistors Rb˜Rn−1 provided in a downstream side of the output transistors TNb˜TNn−1 function to balance the operating current of the current mirror circuit and the N channel transistors TNb˜TNn−1 provided in the downstream side of the resistors Rb˜Rn−1 are switching transistors, which are ON/OFF controlled by the display data Do˜Dn−1.
The output side transistors TNb˜TNn−1 have drains connected to output terminals 11b of the D/A converter circuit 11 and gate widths (channel widths) of the transistors TNb˜TNn−1 with respect to a gate width (channel width) of the input side transistor TNa correspond to weights, for example, 1, 2, 4,˜, n of respective columns. The gate widths (channel widths) corresponding to the weights of the respective columns are usually formed by connecting a plurality of unit transistors, which are formed as transistor cells in parallel.
An output stage current source 13 is constructed with a drive level shifter circuit 13a and an output stage current mirror circuit 13b. 
The drive level shifter circuit 13a functions to transmit the output current of the D/A converter circuit 11 to the output stage current mirror circuit 13b and is constructed with an N channel MOS transistor TNv. The transistor TNv has a gate connected to a bias line Vb, a source connected to the output terminal 11b and a drain connected to an input terminal 13c of the output stage current mirror circuit 13b. 
The output stage current mirror circuit 13b includes P channel MOS transistors TPu and TPw and P channel MOS transistors TPx and TPy, which constitute an output stage current mirror circuit. The drive current is outputted from a drain of the transistor TPy to the organic EL element 4 through an output pin 9.
Such current drive circuit generates a drive current in column direction or a current on which the drive current is generated by D/A converter circuit according to the reference current Ip. The D/A converter circuit is constructed with a number of transistor cells. Therefore, variation of D/A conversion characteristics of the D/A converter circuit cause variation of output currents of the column pins, resulting in luminous irregularity and luminous variation on the display screen.
Since such luminous irregularity and variation can not be absorbed by regulating the reference current value, it is necessary to provide, in the D/A converter circuit, a regulator circuit for regulating the converted current value. Since such regulator circuit has to be provided for each column pin, the circuit size of the D/A converter circuit has to be increased with increase of the number of elements of the regulator circuit, so that it is difficult to form the current drive circuits in an IC.